In known grinding machines, an abrasive grinding wheel is rotatably mounted upon a wheel head for translation relative to a workpiece, such as a cam shaft, that is ground to a desired size and shape. The workpiece is retained in between a headstock and a footstock, and the wheel head, with the grinding wheel, is translated by a nut and lead screw arrangement. The nut is internally threaded and fits about, and coacts with, the externally threaded lead screw. The nut is secured to the wheel head, and the lead screw is driven by a motor, coupled to the end of the lead screw remote from the nut. The motor, which may be numerically controlled, rotates the lead screw relative to the nut, in either a clockwise or counter-clockwise fashion, and thus linearly translates the abrasive grinding wheel relative to the workpiece.
Previously, the abrasive grinding wheels, which might be 16 inches in diameter and formed of carborundum, would be gradually abraded away, and reduced in diameter, by extended cycles of grinding operations. Over a period of time, the wheels might be reduced by three or four inches in diameter. As the wheels gradually "shrunk", the motor, in accordance with a program, would drive the lead screw to advance the wheel head and grinding wheel toward the workpiece to compensate for such shrinkage. As the lead screw was advanced, the wear attributable to continued operation in a hostile industrial environment, on a factory or job shop floor, would be distributed over a length of lead screw comparable to the extent of shrinkage of the abrasive grinding wheel.
With the advent of more durable, harder, grinding wheels, made out of materials such as CBN (cubic-boron-nitride), the grinding wheels are reduced in diameter almost imperceptibly. CBN grinding wheels grind with great precision, require fewer dressing operations, and infrequent replacement, and thus have met with widespread acceptance. However, utilization of CBN wheels, and other grinding wheels with diamond-like hardness, has intensified the problem of wear in the lead screw and nut assembly for the wheel head and grinding wheel. While wear is inherent in using conventional grinding wheel machines, the significant reduction in grinding wheel size tended to distribute the wear over a significant length of the lead screw; conversely, the minor reduction in diameter of CBN grinding wheels, in the order of fractions of an inch, concentrates the wear on a fragment of the lead screw. Such localization of wear causes premature failure. Repeated, precise translational movement of the wheelhead relative to the workpiece is inhibited, and the exacting tolerances demanded by current engineering requirements for cam shafts, and other mechanical movements, cannot be readily maintained.
Numerous approaches have been made to address the problems of (1) localized wear, (2) excessive friction between the contacting metal surfaces of the threads of the nut and lead screw, and (3) backlash.
One approach has relied upon introducing precisely sized ball bearings between the contacting threaded surfaces and circulating the balls throughout the nut. Such approach has satisfactorily addressed the problems of excessive friction and backlash. However, such approach is prone to premature failure and does not have any damping in the direction of motion.
Another approach has relied upon hydrostatic bearings employing a thin film of fluid between the contacting threaded surfaces; such approach also requires the pressurized distribution of the thin film throughout the nut to avoid metal to metal contact between the cooperating, helically arranged teeth on the nut and lead screw. Pressurized distribution calls for precise machining operations within the metallic body of the nut to form pockets, manifolds, distribution channels, etc. and other fluid flow circuits to deliver the appropriate quantities of fluid to the desired locations.
Yet another approach has focused upon casting an epoxy material about a section of the lead screw, or a master form, that replicates such section of the lead screw. The epoxy is treated with special fillers that increase its strength, lubricity, and wear characteristics, while reducing its usual brittleness. After the casting has been allowed to cure, usually at room temperature and pressure, the casting is removed from the lead screw, and retained within a nut housing. When the lead screw is subsequently advanced relative to the nut, the internally threaded nut closely conforms to the configuration of the lead screw, with attendant reductions in frictional losses and misalignments.
The utilization of epoxy materials, such as castable polymers, is discussed in detail in U.S. Pat. No. 4,790,971, granted Dec. 13, 1988, to Ross A. Brown et al, and in U.S. Pat. No. 5,152,948, granted Oct. 6, 1992, to Kevin J. Lizenby; both patents are assigned to TranTek Inc. of Traverse City, Mich.
Each of the foregoing proposed approaches has proven to be deficient in some manner, such as cost, complexity, operational characteristics, etc., and the need for a solution to all of the problems noted above, such as localized wear, excessive friction, and backlash, remains unfulfilled.